Conventionally, wire electrical discharge machines are used for metal die sinking, cutting and the like. When processing is carried out in a wire electrical discharge machine, machining swarf are suspended in the processing liquid; in order to remove the machining swarf and separate them from clean liquid, a filter medium unit in which a pleated filter medium is incorporated is set up. In this filter medium unit, a pressure gauge is set up to monitor the pressure of the processing liquid that passes through the filter medium unit. When the pressure of the processing liquid entering the filter medium unit approaches the vicinity of 195 kPa, this pressure gauge sends a signal to the main body of the wire electrical discharge machine, and the operation shuts off automatically to exchange the filter medium unit.
Moreover, when the processing liquid pressure is below 195 kPa, if the filter medium incorporated in the unit cannot withstand the pressure and bursts into the state of being unable to filter the machining swarf, the operation is to shut off automatically.
When the filter medium incorporated in the unit bursts at a pressure of 195 kPa or lower, precision of die sinking declines and an extension of operation time caused by the stoppage of the wire electrical discharge machine occurs resulting in a significant decline in operating efficiency. In order to prevent such problems from occurring, as a required property for the filter medium, bursting strength of 200 kPa or higher when wetted in water for 30 minutes or longer at 30° C. as described in JIS L 1096 is used as an index.
Nevertheless, due to the recent improvement in the performance of wire electrical discharge machines, the pressure of some of the processing liquids rises up to 297 kPa. Consequently, problems with bursting caused by the use of the conventional filter medium have been reported.
Additionally, generally used for processing liquid filter media of wire electrical discharge machines and the like are polyester non-woven fabrics and the like. However, these polyester non-woven fabrics and the like have a high basis weight and a thick sheet thickness. As a result, when pleating is performed and the filter medium is incorporated in a unit, a problem arises that the total filter medium effective area is small.
On the other hand, in the market, while the filter medium basis weight of 110 g/m2˜300 g/m2 remains to be the main stream, the tendency is to move toward the direction in which the filter medium is pleated and incorporated in a unit by decreasing the basis weight, and the filter medium life is increased by increasing the total filter medium area. However, decreasing the sheet thickness and lowering the basis weight result in lower bursting strength caused by water wetting. Moreover, when the sheet thickness is decreased while keeping the basis weight as it is, the filter medium becomes very tight; resistance to filtering becomes high and the filter medium life is reduced.
Moreover, generally polyester non-woven fabrics and the like are comprised of heat fused fiber and principal fiber. In this case, although the intersection between the heat fused fiber and the principal fiber which is in contact with the heat fused fiber are bonded, other intersections where the heat fused fiber is absent are not adhered together. Consequently, the strength of non-adhered portions becomes weak resulting in a problem that the total filter medium bursting strength when wetted in water becomes weak.
In order to solve these problems, a proposal on partial thermo-compressional bonding so that bursting strength at the time of water wetting becomes 5.0 kgf/cm2 or higher is made (for instance, Patent Reference 1). However, thermo-compressional bonding gives rise to a shortcoming that filter medium voids collapse to lower the filter efficiency. Moreover, since it is a dry type production process, variations in basis weight are wide, and the filter efficiency fluctuates.
Additionally, a proposal is made to use a filter medium for liquid filtration in which a filter medium is integrated with a backing layer after paper making, and to apply a binder to the backing layer so as to have wet tensile strength in the cross direction of 0.98 kN/m or higher (for example, Patent Reference 2). Nevertheless, applying a binder only to the backing layer renders the filter medium void structure non-uniform; consequently, filtering cannot be done uniformly inside the filter medium, and the initial filter precision fluctuates markedly.
Moreover, when organic synthetic fibers are used to make paper with the wet type paper-making, a proposal is made to employ surface active agents and thickeners to improve the formation (Patent Reference 3). If surface active agents are used, they remain in the sheet; when the sheet is used for a filter medium for liquid filtration, while the sheet wet-ability improves, the strength decreases significantly. At the same time, the surface active agents separate to cause foaming, and affect processing precision tremendously. Moreover, if large quantities of a thickener are used, peeling from paper machine wires is impaired causing sheet non-uniformity and coarseness inside the sheet increases, hence variations in the initial filter precision increase. As the strength is low in the coarse portion, a problem with breakage before reaching a prescribed pressure occurs.
Furthermore, a proposal is also made to coat phenolic resins on polyester non-woven fabrics and the like after drying to increase bursting strength (for example, Patent Reference 4). When phenolic resins are coated, however, mere coating and drying base materials do not increase bursting strength when wetted in water. In order to increase bursting strength when wetted in water, after phenolic resins are coated and dried, usually heat of 100° C. or higher must be applied for several minutes at a separate step. Therefore, an addition of the heat curing process or modification of the existing process is required; hence it takes equipment and much expense in time and effort.
Moreover, a filter medium using fibrilized organic fibers is proposed. If such fibers are used, however, in order to maintain strength in water, the filter medium becomes very tight, resistance to filtering increases and filter medium life becomes short. In order to solve this problem, a pasted two layer structure is used and the thickness of the dense portion is thinned (Patent Reference 5). However, pasting tends to cause interlayer peeling due to poor adhesion and, additionally, an addition of the pasting process and the like or modification is required.
Patent Reference 1: JP 11-165009 A
Patent Reference 2: JP 2003-38918 A
Patent Reference 3: JP 9-155127 A
Patent Reference 4: JP 7-26499 A
Patent Reference 5: JP 4-313313 A